include

include

using namespace std;

int main() {

int n;
cin >> n;
int nums[n];
unordered_set<int> numSet;

for (int i = 0; i < n; i++) {
	cin >> nums[i];
	numSet.insert(nums[i]);
}

int count = 0;
unordered_set<int> validSums;

for (int i = 0; i < n; i++) {
	for (int j = i + 1; j < n; j++) {
		int sum = nums[i] + nums[j];
		if (numSet.count(sum) && !validSums.count(sum)) {
			validSums.insert(sum);
			count++;
		}
	}
}

cout << count << endl;
return 0;

}